7SR210 & 7SR220 Applications Guide
Page 38 of 48 ©2018 Siemens Protection Devices Limited
5.2 Current Transformer Supervision (60CTS)
When a CT fails, the current levels seen by the protection become unbalanced. A large level of NPS current is
therefore detected - around 0.3 x In for one or two CT failures. However this condition would also occur for a
system fault. To differentiate between the two conditions, the element uses NPS voltage to restrain the CTS
algorithm as show in the accompanying table.
NPS Current NPS Voltage Decision
> Setting > Setting System Fault
> Setting < Setting CT Failure
Table 5-2 Determination of CT Failure (1 or 2 Phases)
Following a CT Failure, there should be little or no NPS voltage. Perhaps 0.1 x Vn as a maximum.
Operation is subject to a time delay to prevent operation for transitory effects.
A 3-phase CT failure is considered so unlikely (these being independent units) that this condition is not tested for.
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